Since a polyester film has excellent producibility, mechanical strength and dimensional stability, it has heretofore been considered to be substitutable for TAC. However, when the polyester film is used as a support of a photographic material of the most popular roll film, it strongly curls and the curl of the film strongly remains even after development. As a result, the handling of the curled film is difficult and troublesome after development. Therefore, despite of the above-mentioned excellent properties, the use of the polyester film in the field of photographic materials has heretofore been limited.
For reducing the curling property of a polyester film, JP-A-51-16358 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method in which a polyester film is heat-treated at a temperature lower than its glass transition temperature by 5.degree. C. to 30.degree. C.
Some kinds of a polyester film have a higher refractive index than that of TAC or PET, which causes light fogging due to so-called light piping. This problem has been required to be solved.
On the other hand, the use of photographic materials is widely diversified in recent years, and the technology for rapid feeding in a camera or the like during picture-taking, elevation of image magnification, and reduction of the size of picture-taking devices has advanced noticeably. With this advanced technology, the support of photographic materials must have high strength and high dimensional stability and be as thin as possible.
In addition, with the great reduction of the size of picture-taking devices, the demand for reducing the size of film-housing cartridges has also become great. TAC supports cannot satisfy the demand due to lack of the mechanical strength.
Reduction of the size of a cartridge involves two problems.
The first problem is that the reduction of the thickness of the film to be in the cartridge is accompanied by lowering of the mechanical strength of the film itself.
The second problem is that the film in a small-sized cartridge with a small-sized spool is strongly curled during storage of the film. For instance, if the exposed film taken out of such a small-sized cartridge is developed in a mini-laboratory automatic developing machine, the film would be curled up during handling, since only one edge of the film is fixed to the leader but the other one is not, so that feeding of a processing solution to the curled up area would be delayed to cause so called "uneven development". In addition, the curled-up film would be crushed in the machine and the film being processed would then be "bent or broken".